Thanks waterloo2005 and BillWho, for asking/answering my question before I even asked it

BillWho wrote:

In ubuntu you can ... add the entry to /etc/grub.d/40_custom if the probe doesn't detect gentoo .

If like me, you want the gentoo entry to appear first, cp 40_custom 07_custom and then modify that to offer the gentoo symlinked kernel. The sections appear in numerical order, so gentoo now jumps the queue. It may or may not also appear lower down, depending on the probe.

Setting gentoo as the default boot option in the Ubuntu boot menu

Once 07_custom is working, you may find the side effect was to set gentoo to be the default, because it is now always the top entry in the menu. Indeed, if you never changed the Ubuntu defaults, this is likely to happen as soon as you use the 07_custom entries.

To alter the defaults, edit /etc/default/grub, changing the following entries to suit your needs (these are my preferences). The menu entries are numbered from zero.

GRUB_DEFAULT=0
GRUB_TIMEOUT=5

Then, to make that effective, in an Ubuntu terminal or console

sudo update-grub

It is harder to set Gentoo as the default using the 40_custom entry, as you do not know exactly how many items will be in the menu above. And anyway, doesn't Gentoo belong at the top